A necessary target in realizing alternative energy sources, in particular in the transportation sector, is hydrogen storage for controlled delivery to an energy-producing fuel cell. Chemical hydrogen storage has been dominated by ammonia borane (H3B—NH3, or “AB”), which is a desirable material due to its high gravimetric capacity of hydrogen (19.6 wt %) and low molecular weight (30.7 g mol−1). In contrast to the loss of H2 from C2H6, which is substantially endothermic, AB has both hydridic and protic moieties, yielding a material from which H2 can be readily released. As such, a number of publications have described H2 release from amine boranes, yielding various rates depending on the method applied.
Spent hydrogen fuel composition depends on the dehydrogenation method from which it was produced. To date, the majority of efforts have employed metal-based catalysis. Metal-based catalysts have produced the fastest rates for a single equivalent of H2 released from AB and up to 2.5 equivalents of H2 can be produced within 2 hours. The predominant, and most desirable, product of hydrogen generation from ammonia borane via metal-based catalysis is polyborazylene (“PB”).
The viability of any chemical hydrogen storage system is critically dependent on efficient recyclability, but reports on the latter subject are sparse, invoke the use of high energy reducing agents, and suffer from low yields. Previous methods of regeneration of AB from polyborazylene involve multiple steps and require the use of a metal-containing reducing agent and reagents such as benzenedithiol. This increases the cost of industrial-scale production, due to the relatively high molecular weight of the reagents, the cost of disposal of the reagents and byproducts, etc. A need exists, therefore, for a simpler and more effective process for regenerating ammonia borane from H2-depleted AB, in particular, polyborazylene.